SKETCH OF M. ARAQO. 261 



of gases, and the determination of the relative densities of the air and 

 mercury, from which they were able to calculate theoretically the con- 

 stant of the barometric formula. The two friends next were interested 

 in the scheme of continuing the measurement of the arc of the merid- 

 ian from the shore-line at Barcelona, where Delambre and Mechain 

 had left it, to Majorca, and thence to Formentera. In the execution 

 of this enterprise it became necessary to set up a beacon-light in the 

 Island of Ivica, and to observe it from two points in Spain, forty 

 leagues off. Arago established himself upon a rock called Desierto 

 de las Palmas, the summit of which hardly afforded room enough for 

 his tent and instruments. On account of the distance, the frequency 

 of fogs, and uncertainty as to the exact direction, it was six months 

 before he could get sight of the beacon. As soon as the measurements 

 were made, Biot took the first results to France, leaving his colleague 

 to finish the work at Majorca and Formentera. Then began for 

 Arago a series of exciting adventures. War had been declared with 

 Spain in 1808, and the Majorcans sought to arrest him as a spy. He 

 had barely time to disguise himself, and, gathering up the papers con- 

 taining his observations and his instruments, to take refuge on the 

 vessel which had brought him to the island. Thence he escaped, 

 with the aid of the ship-captain, to the citadel of Belver, where he 

 passed several months, substantially a prisoner, in making his calcula- 

 tions. He was at last permitted to go to Algiers, where he embarked 

 on a vessel of the regency under the name of a Hungarian merchant, 

 with a false passport, along with a crowd of Mussulmans and rene- 

 gades, to which were added two lions and a family of monkeys which 

 the dey was sending to his ally the Emperor of France. The vessel 

 was taken close upon Marseilles by a Spanish corsair, " which con- 

 ducted its prize to Palamos and took Arago to a country where he was 

 only too well known and from which he had nothing good to expect." 

 He suffered great privations till the dey was informed of the state of 

 affairs, and bullied Spain into setting the lions, monkeys,' ships, and 

 philosopher again on the way to Marseilles. They had reached that 

 port when a mistral suddenly arose and sent the vessel to the coast of 

 Sardinia, and thence to Bongie, in Algeria, where the new dey, not so 

 friendly as his predecessor, was disposed to hold Arago as a prisoner. 

 Finally, he was allowed to return to France, running a new danger 

 from pursuit by an English cruiser, and at length to receive a welcome 

 from his mother, " who thanked God for having preserved his life, 

 after having had masses said for the repose of his soul ; he returned 

 with the triple consecration of having encountered danger, done his 

 duty, and attained a scientific success beyond his anticipations." 



In 1809 the phenomena of optics engaged especial attention, and 

 Arago entered upon the study with the ardor of his nature and his 

 age. He engaged in the investigation of the polarization of light, 

 which no one had as yet been able to explain, but of which he reached 



